1. Technical Field
The present invention relates to a method and apparatus for coating or microencapsulating solid particles or viscous liquid droplets. More particularly, the present invention relates to improvements in such methods and apparatus which provide encapsulation techniques and effects which are unprecendented in the prior art.
2. Discussion of the Prior Art
Coating or microencapsulation of solid particles or liquid droplets is widely employed to protect coated substances from environmental effects and/or control their release time and/or confer improved handling characteristics. Typical products which are coated or microencapsulated are drugs, pesticides, dyes, etc..
Numerous coating or microencapsulation techniques have been employed in the prior art, many of which are described in the Encyclopedia Of Chemical Technology, third edition, volume 15, pages 470-493 (1981), John Wiley and Sons. By and large, these techniques suffer from one or more important disadvantages, including: high cost; inapplicability for coating particles smaller than 200 micrometers in diameter; complexity; long contact time between the core and coating materials prior to solidification of the coating material; inability to achieve wetting and coating of the core particles with the desired coating material; inefficient separation of coated particles from unused coating material and inefficient usage or wastage of coating material. Also important in many methods are the tendency for the coated particles to agglomerate and the limited choice of wall materials. There are severe cost disadvantages to most methods because they are batch processes difficult to operate on large commerical scale and because they must employ a solvent for the coating and are unable to use melted coating materials, which require no solvent removal or handling facilities.
There have been a number of attempts in the prior art to provide coating techniques which are devoid of the aforesaid disadvantages. For example, in U.S. Pat. No. 4,386,895 (Sodickson), there is disclosed a rotating apparatus having radially-extending conduits from which hollow needles project radially outward into a reservoir of jelling material. As the apparatus spins, liquid core material is urged by centrifugal force through the conduits and needles. The liquid core material is formed into droplets at the distal ends of the needles, and the droplets are centrifugally thrown into a layer of the gelling material which forms on the outer reservoir wall due to the centrifugal forces produced by rotation. The droplets of liquid core material are thusly encapsulated by the gelling material. This technique works well for its intended purpose. However, it is limited to use with liquid as a core material (i.e., it cannot be used to microencapsulate solid particles) and the minimum size droplet that can be coated depends upon the inner diameter of the needle. As to the latter limitation, there are practical limitations on minimum needle size, particularly when viscous core liquids must flow therethrough.
In U.S Pat. No. 2,955,956 (Baugh et al.), a rotating disc or table is disposed below a feed pipe through which a slurry composition of coating material is fed. The slurry is spread over the spinning disc surface to form a thin film of the coating material thereon. An annular flow of solid granules is permitted to impinge upon the film on the disc surface, whereupon the granules are coated with the coating material. The coated granules are thrown or are permitted to fall from the rotating disc and are solidified by dry warm gas directed at the falling granules. A second annular flow of granules is directed onto the rotating film to scavenge the unused film and assure that all of it is utilized. Again, this technique is satisfactory for a limited purpose, namely coating granules, such as salt, with additives, but it cannot be readily employed to coat liquid droplets. Moreover, since the granules in the scavenging outermost annular flow cannot possibly be coated to the same extent as granules in the innermost flow, it is not possible with this technique to achieve uniform coating of all of the granules. Therefore, the Baugh et al. technique is more suitable for wide dispersion of additives onto the surface of granules than it is for coating particles.
British Pat. No. 1,090,971 to Wilson, et al., discloses a method of microencapsulating solid particles by forming a dilute suspension of the particles in a dilute solution of a resinous coating material in a volatile liquid, causing the suspension to impinge on a spinning disc whereby the dilute suspension is dispersed as a spray consisting of atomized coating solution and microencapsulated particle droplets, the spray of droplets then being exposed to steam at temperatures above the boiling point of the coating solvent which volatilizes the unwanted liquid solvent so as to leave coated particles plus particles of pure coating of the same size. The process, however, requires a feedstock solution having a very low percentage content of particles to be coated, involves the high temperature removal of a large amount of unused feedstock liquid by volatilization, and does not permit separation by sizing of coated particles from particles of pure coating material.